The development of planar transmission media in the early 1950's had a major impact on microwave circuit and component packaging technology. The engineering of the microwave printed circuit and the supporting analytical theories for stripline and microstrip occurred at a rapid pace. The early years of stripline circuit design were devoted almost entirely to the design of passive circuits such as directional couplers, power dividers, filters, and antenna feed networks. Early implementations were housed in bulky metal housings and connected by coaxial connectors.
To reduce size and weight, case-less and connector-less couplers were developed. These later implementations were sometimes referred to as “filmbrids” and included laminated stripline assemblies bonded together by fusion, or with thermoplastic or thermoset films. Further refinements continued in areas such as the dielectric materials used in these devices and the microwave-circuit fabrication process itself. A historical perspective on the development and applications of microwave integrated circuits, can be found in “Microwave Integrated Circuits—An Historical Perspective”, H. Howe, Jr., IEEE Trans. MTT-S, Vol. MTT-32, pp. 991-996; September 1984; and “Microwave Printed Circuits—The Early Years”, R. M. Barrett, IEEE Trans. MTT-S, Vol. MTT-32, pp. 983-990; September 1984.
Stripline and microstrip components have been integrated for various applications in housings and packages, as well as monolithically on a common substrate. Methods of integration and packaging affect the system interface and installation, as well as the module's ability to handle post-processing temperatures (i.e., post-manufacture of the stripline or microstrip component), and the module's operating thermal management ability (i.e., its heat transfer ability). Common techniques for integrating components call for bonding them together using, e.g., epoxies, adhesives and solder. In some cases, a module that uses epoxies, adhesives, solder and/or other bonding agents will be subject to subsequent processing steps exposing the module to high temperatures or other processing conditions. These subsequent processing steps must be compatible with the bonding agent and material used in forming the modules. For example, when a module is formed using conventional epoxies, adhesives, and solders, high temperature post-processing may need to be avoided as it may cause deterioration in module bonding.
Planar signal coupling and processing modules that can be subject to, and retain their integrity under, a wider range of manufacturing processes are desired. Accordingly, there exists a need to integrate coupler circuitry, DC blocking structures, impedance matching networks, bias decoupling structures and RF load terminations into a modular planar structure that will be able to withstand, e.g., high temperature processing or other processing steps that the module may be subject to after its manufacture. Such processing may occur, e.g., when the microwave module is integrated with other components in a circuit assembly. Furthermore, there exist a need to have coupling circuitry that can be easily customized by the addition of components post-manufacture of the coupling assembly.